1. Field of the Invention
The present invention relates generally to asphalt mixing apparatus and, more particularly, but not by way of limitation, to asphalt mixing apparatus which may utilize particulated, previously laid asphalt mix in the preparation of fresh hot mix asphalt.
2. Background of the Invention
While hot mix asphalt is a necessary commodity in an industrial society, its use for such purposes as paving roadways, parking lots and the like, has not always been without excessive environmental and energy costs. Asphalt is produced from aggregate and asphaltic oil which are heated and mixed in a rotating drum and heating of the aggregate is accomplished by means of a burner at the input end of the drum, such burner producing a flame in portions of the drum near the input end of the drum to dry the aggregate prior to mixing the aggregate with the asphaltic oil. It is common to draw excess air through the drum by means of a fan or the like and such air can entrain fine particles in the aggregate so that the fines are discharged into the atmosphere as the air is discharged from the drum. Sources of excessive energy costs are inefficiency in heat transfer from the flame to the aggregate and burning of the asphaltic oil by the flame.
The drum of the asphalt mixing apparatus is disposed on a slant and flighting is mounted about the inner periphery of the drum to lift and drop the aggregate and asphalt across the interior of the drum, to form a veil of falling material across the drum. This veiling serves several purposes. The aggregate is dropped through the flame to enhance heat transfer from the flame to the aggregate and the lifting and dropping of aggregate while introducing asphaltic oil into the drum serves to mix the aggregate and oil to form the asphalt. Moreover, the veiling also advances the aggregate and asphalt toward the lower, or discharge, end of the drum. That is, since the drum is disposed on a slant, the flighting drops aggregate and asphalt forwardly of the point where the asphalt or aggregate was picked up by the flighting.
It has been recognized that the interplay of heated air passing through the drum and the veiling of material across the drum can be utilized to reduce the problem of fines being discharged into the atmosphere and to increase the efficiency of fuel utilized to heat the aggregate from which the asphalt is made. Thus, in U.S. Pat. No. 3,940,120, issued Feb. 24, 1976 to Miller, circumferential baffle rings are disposed between rows of flighting to reduce the discharge of fines. The baffle rings have the effect of bunching up asphalt or aggregate in the vicinity of the rings and of deflecting the air stream through the drum, such air stream entraining fines, into the veil of aggregate or asphalt across the drum. Similarly, the aforementioned U.S. patent application, Ser. No. 896,512, teaches that fines in the discharge from an asphalt mixing apparatus can be reduced and heating of aggregate can be made more efficient by establishing zones of different veil density in different parts of the drum.
While the teachings of the above cited patent and patent application have made valuable advances in the state of the art of asphalt production, it is important that improvements in fines discharge reduction and fuel efficiency continue. Concern for the environment can only result in more stringent governmental regulations of fines discharge and the need to conserve fuel stocks is well known.